Some manufacturers of automotive internal combustion engines require that each assembled engine be tested dynamically to minimize vibration that may be transmitted from an unbalanced engine to the vehicle in which the engine is installed. Some engine manufacturers balance each individual component prior to its inclusion in the final assembly with other components on the theory that, if each component is balanced, an assembly of such components also will be balanced. All components, however, are manufactured to tolerances. It is possible, therefore, that the tolerances may accumulate in such manner as to produce a condition of imbalance in one or more of the subassemblies, thereby resulting in the generation of unacceptable vibrations even though all of the component parts of the assembly are within the tolerance applicable to each such part.
Other engine manufacturers assemble all of the component parts in the engine and thereafter perform a balance test. This kind of balance testing, without ensuring that all of the components themselves are within tolerance limits, runs the risk of including in an assembled engine grossly imbalanced component parts. This can require an excessive amount of balance correction if an engine is to meet specifications.
The presence of an unbalanced mass in an automotive engine of the kind having a rotary crankshaft and reciprocating pistons normally generates two vibration frequencies. The truly rotary elements, such as the crankshaft and a portion of each connecting rod, as well as the flywheel and the front damper/pulley assembly, generate a first order vibration once during each revolution of the crankshaft. Reciprocating pistons, piston pins, and the remaining portions of the connecting rods generate second order vibrations twice for each revolution. For example, an engine running at a speed of 1200 rpm will generate 20 HZ first order vibrations and 40 HZ second order vibrations if the respective rotary and reciprocating components are not balanced.
The conventional method of measuring engine imbalance is to mount the engine on flexures at both the front and rear of the engine which permit displacement of the engine due to imbalance in each of two horizontal planes perpendicular to the crankshaft. Horizontal velocity or displacement sensors located at each end of the engine measure the horizontal movement created by the forces due to imbalance as the crankshaft rotates. Typically, these sensors measure only first order imbalance forces which are correctable externally of the engine without necessitating disassembly thereof.
Since the conventional balance measuring system referred to above is responsive only to first order horizontal vibrations, it is insensitive to second order vibrations which occur in V-type engines, for example. Consequently, it is not uncommon for the correction of first order imbalance to cause an increase in second order imbalance, thereby resulting in excessive vibrations transmitted through the engine mounts to the vehicle frame. Thus, without measuring both vertical and horizontal vibration components in both the front and rear planes, reciprocating automotive engines cannot always be balanced accurately.
If the workpiece being tested for balance is an internal combustion engine having reciprocating pistons, the movements of the pistons are accompanied by compression and expansion of the gas within the cylinders. The compression of gas imposes loads on the moving parts of the engine, thereby making balance measurements more complex than they would be if no compression loads were present.